The present invention relates to instrumentation that simultaneously measures signals over a band of frequencies, and more particularly to filter equalization for such instrumentation using magnitude measurement data to improve the accuracy of frequency and time domain measurements.
In modern telecommunications information is transmitted digitally by many modulation techniques. These techniques include modulating frequency, phase and/or magnitude. As modulation schemes have become more complex, the pressure on the telecommunications industry to provide equipment with greater accuracy has increased. Typical communications standards require good amplitude flatness and phase linearity to meet performance targets, such as bit error rate. In order to determine the accuracy of the telecommunications equipment, measurement instrumentation is required with even greater accuracy. However such measurement instrumentation contains filters that affect the magnitude and phase of different frequencies in a different manner, i.e., at one frequency the magnitude of the signal may be attenuated greater than at another frequency within the frequency passband while the phase or delay of the signal through the filter may also be affected at different frequencies. Ideally the filter should pass all frequencies within its passband with no attenuation or equal attenuation and the delay through the filter should be the same for all frequencies so there is no relative phase change from frequency to frequency within the filter passband.
For lower frequencies a current technique provides a calibrated source that outputs a plurality of frequencies in a combined signal, i.e., a signal having a comb-like frequency characteristic. The signal, after passing through several stages of filtering, is digitized and the magnitude and phase are measured and compared to known ideal results. An inverse filter is then provided to process the digitized output based upon the measurement results so that the resulting output conforms to the known ideal results.
For intermediate frequency (IF) channel equalization on radio frequency (RF)/microwave instruments, the design of the calibrated source or stimulus signal is key. For the low frequency band a repetitive broadband signal, such as a pseudorandom noise (PRN) signal, may be used as the stimulus source and readily implemented with a linear feedback shift register followed by a fast response flip-flop. The repetitive signal exhibits the comb-like spectrum. There are known magnitude and phase relationships among the spectrum lines. The channel frequency response to this stimulus signal is first measured so that the overall IF channel frequency response may be evaluated at the spectrum lines. The IF channel frequency response is finally obtained by removing the frequency response of the stimulus signal. In order to maintain good signal-to-noise ratio (SNR) for the spectrum lines, the useful part of the PRN spectrum is usually chosen to be the same order of magnitude as the signal bandwidth of the instrument.
For high frequencies, however, the PRN signal at a frequency band of interest generally does not have sufficient power to achieve the desired performance since the amplitude of the spectrum follows a sin(x)/x envelope. Other equalization sources, such as an orthogonal frequency division multiplexing (OFDM) modulation signal, may be used instead. Compared to the PRN approach, this second approach requires much more in hardware resources, such as a digital-to-analog converter (DAC), mixer and local oscillator (LO). During the manufacture or service calibration the frequency response both in magnitude and phase of the stimulus signal needs to be measured. Source calibration on up-converted OFDM signals is particularly challenging due to a lack of well specified signal generators at high frequencies. In other words for equalizing high frequency bands of a measurement instrument there is no readily available phase-calibrated source. As a result measurement errors at the high frequency bands may reach 30% or greater, which greatly exceeds the measurement accuracy required to assure that telecommunications equipment is operating correctly to provide an unambiguous communication signal.
What is desired is a technique for equalizing high frequency bands of a measurement instrument that accounts for both magnitude and phase with an accuracy greater than that required by the equipment being measured.